# std::unary_function

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
template< typename ArgumentType, typename ResultType > struct unary_function; |  | (deprecated in C++11) (removed in C++17)

`std::unary_function` is a base class for creating function objects with one argument.

`std::unary_function` does not define operator(); it is expected that derived classes will define this. `std::unary_function` provides only two types - `argument_type` and `result_type` - defined by the template parameters.

Some standard library function object adaptors, such as std::not1, require the function objects they adapt to have certain types defined; std::not1 requires the function object being adapted to have a type named `argument_type`. Deriving function objects that take one argument from `std::unary_function` is an easy way to make them compatible with those adaptors.

`std::unary_function` is deprecated in C++11.

### Member types

Type | Definition
`argument_type` | `ArgumentType`
`result_type` | `ResultType`

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <vector>

    struct less_than_7 : std::unary_function<int, bool>
    {
        bool operator()(int i) const { return i < 7; }
    };

    int main()
    {
        std::vector<int> v(10, 7);
        v[0] = v[1] = v[2] = 6;

        std::cout << std::count_if(v.begin(), v.end(), std::not1(less_than_7()));

        // C++11 solution:
        // Cast to std::function<bool (int)> somehow - even with a lambda
        // std::cout << std::count_if(v.begin(), v.end(),
        //     std::not1(std::function<bool (int)>([](int i) { return i < 7; })));
    }

Output:

    7

### See also

function (C++11) | copyable wrapper of any copy constructible callable object (class template)
move_only_function (C++23) | move-only wrapper of any callable object that supports qualifiers in a given call signature (class template)
ptr_fun (deprecated in C++11)(removed in C++17) | creates an adaptor-compatible function object wrapper from a pointer to function (function template)
pointer_to_unary_function (deprecated in C++11)(removed in C++17) | adaptor-compatible wrapper for a pointer to unary function (class template)
binary_function (deprecated in C++11)(removed in C++17) | adaptor-compatible binary function base class (class template)

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